1. Field of the Invention
The present invention relates to electronic speed control circuitry for dc motors, and particularly to such circuitry useful in conjunction with radio controlled models.
2. Description of the Prior Art
The use of electric motors in radio controlled model aircraft, boats and autos has gained widespread acceptance amongst modelers. Such motors are almost silent, as compared to glow-plug type combustion engines, are cleaner and easier to start, run and maintain, and require no cumbersome liquid fuel supply. They permit implementation of electronic speed control in flight or during travel of a model boat or auto.
Most radio control systems for models employ a pulse width modulation system. The radio control receiver includes a digital decoder which produces an output pulse train in which successive pulses occur at a 16 msec frame rate. The duration of each pulse typically is in the range of from 1 msec to 2 msec, depending on the position of the control throttle at the radio control transmitter. With some transmitters, the pulse width increases when the throttle lever is advanced. Thus, a pulse of minimum width (1 msec) indicates a closed throttle, and a maximum pulse width (2 msec) indicates full throttle or maximum speed. In other transmitters, the pulse width decreases (e.g., from 2 msec to 1 msec) as the throttle lever is advanced.
A principle object of the present invention is to provide circuitry for controlling the speed of a dc motor in a radio controlled vehicle in response to the pulse width modulated signal obtained from the radio control receiver. Another objective is to provide such a controller that is operative selectively with systems in which the pulse width either increases or decreases to achieve an increase in motor speed.
A further object of the present invention is to provide an electronic speed controller that is operative with one or more dc motors having a wide range of power. This is important, since typical motors used with model vehicles have a wide range of operating voltages and currents. Thus, the motors may range from a low of 3 volts at 1 ampere (3 watts) to a high of 36 volts at 25 amperes (900 watts). For any motor size, it is important to minimize the power drain of the control circuitry, so that battery lifetime between recharging, and hence the flight time of the model, is maximized. Another object of the present invention is to provide a speed controller in which battery drain is minimized.
By contrast, prior art speed control techniques used power very inefficiently, and thus drained down the limited battery power at excessively fast rates. One such inefficient system employed a conventional digital pulse mechanical servo to move the arm of a rheostat placed in series with the motor. When the motor voltage is reduced to one-half, the rheostat burns up in heat an equal amount of energy. Thus, if the motor is run at low speeds, one-half or more of the battery power is totally wasted. Flight time is reduced concomitantly. Furthermore, a series resistance type controller exhibits poor speed regulation of the motor as the power is diminished when torsional load is applied, because the increasing current requirements cause a further voltage drop to the motor.
Another prior art speed control method employed a digital pulse responsive mechanical servo coupled to a potentiometer. Like the servo-controlled rheostat, this technique has the inherent unreliability of requiring mechanical connection to the variable resistance. The potentiometer typically was used to drive a dc transistor amplifier, usually an emitter follower. Better speed regulation was achieved because a fixed voltage is generated for a certain pulse width output from the radio control receiver. However, the problem of dissipated heat loss still exists, and is compounded since there is a power loss even when the motor is full on.
This full-on power loss has two components. The first concerns the power loss across the transistor which is connected in series between the battery and the motor. This slight loss is called the Vce saturation voltage drop between the collector and the emitter of the transistor. The other power loss, which is far more significant, is related to the gain of the output transistor. With practical power transistors, a current gain of 10 to 20 is nominal. Thus, if a 20 ampere motor is controlled by the transistor, an input current of e.g., 1 ampere is required just to turn the transistor fully on.
To achieve minimum Vce saturation voltage drop, a common emitter configuration is preferred. In such a circuit, the base to emitter current does not flow through the motor. Thus, the input power required to control the transistor represents a power loss, since it does not add to the current flowing through the motor. The amount of the this power loss is determined by the Vbe drop between the base and emitter of the output power transistor. Typically, this Vbe drop is 1 volt.
If a base current of 1 ampere is required to turn the motor-control transistor fully on, and this transistor has a Vbe drop of 1 volt, 1 watt is dissipated in the transistor. However, in the prior art potentiometer control system, the desired base current was achieved by using a resistor to drop the battery supply voltage to 1 volt at the desired base current. If a 36 volt motor is employed, then the drop across the resistor is equal to the 36 volts from the battery minus the 1 volt Vbe drop, at the required base current of 1 ampere. That is, 35 watts of power are dissipated by the resistor, even when the power transistor is fully saturated and the motor is running at full speed.
In the example just given, the base drive required by the transistor is 1 watt, equal to the Vbe drop of 1 volt times the base drive current of 1 ampere. However, using the dropping resistor technique to supply this power, 35 watts of power were dissipated. This is roughly a base drive efficiency of 3%. Obviously, this prior art technique of controlling motor speed in response to the decoder pulse output from a radio control receiver is highly inefficient. A further object of the present invention is to provide a totally electronic circuit for controlling the dc motor speed in which high efficiency is achieved in providing the base drive to the output power transistor, with a corresponding substantial reduction in battery power loss. Yet another object of the present invention is to provide such a controller which is totally electronic in operation and which does not employ a mechanical servo.
Some electronic controls have been developed in the past which vary the duty cycle of the battery potential applied to the dc motor in response to a variation in the control pulse width. In these systems, however, the time between pulses applied to the motor is equal to the frame rate, which as noted above, is approximately 16 msec (the time between consecutive information pulses each of which has a pulse width in the range of from 1 msec to 2 msec). These prior art pulse stretching control systems are frame rate sensitive. That is, if the frame rate should vary, different motor speeds would result even though the control pulse width remains at a constant value. Such frame rate sensitivity is undesirable for several reasons. For example, individual radio control transmitters, or those from different manufacturers, may have a frame rate that varies according to the control positions. This may result in an interaction between different control channel functions. For example, a change in rudder position may effect the frame rate and thus change the motor speed.
Another problem associated with pulse width modulation systems operating at the frame rate is that the motor tends to run rough at low speeds. Further, the motor brushes wear out faster, because application of current to the motor in bursts at the frame rate is analogous to starting a motor repetitively under load conditions. This results in high transient currents that may be in excess of the continuous current capability of the brushes. Yet another object of the present invention is to provide an electronic speed controller which does not switch current to the motor at the frame rate, and which is frame rate insensitive.
Still a further object of the present invention is to provide a circuit for controlling both the speed and the direction of rotation of a dc motor in response to a pulse width modulated signal from a radio control receiver. The circuit is particularly useful to control the electric drive motor of a model boat or car. It facilitates the remote selection of forward or reverse direction, in addition to controlling the vehicle's speed.